Incertaintypesofarterialdisease,particularlyhypertension,thesubendotheliallayersofvesselstendtodegeneratelocally,andsmallregionsoftheendotheliummaylosetheirnormalsupport.Theviscousdragonthearterialwallmaycauseatearbetweenanormallysupportedregionandanunsupportedregionoftheendotheliallining.Bloodmaythenflowfromthevessellumenthroughtheriftintheliningandbecomedissectedbetweenthevariouslayersoftheartery.Suchalesioniscalledadissecting aneurysm. Itoccursmostoftenintheproximalportionsoftheaortaandisextremelyserious.Onereasonforitspredilectionforthissiteisthehighvelocityofbloodflow,withassociatedlargeshearratevaluesattheendothelialwall.Shearstressatthevesselwallalsoinfluencesmanyothervascularfunctions,suchasthepermeabilityofthevesselwallsbylargemolecules,thebiochemicalactivityofendothelialcells,theintegrityoftheformedelementsinblood,andbloodcoagulation.Anincreaseinshearstressontheendothelialwallisalsoaneffectivestimulusforthereleaseofnitricoxide(NO)fromvascularendothelialcells;NOisapotentvasodilator(seethesection“ •Fig. 17.7 Therelativeviscosityofwholebloodincreasesataprogressivelygreaterrateasthehematocritratioincreases.Foranygivenhematocritratio,theapparentviscosityofbloodislowerwhenmeasuredinabiologicalviscometer(suchasahindlegbloodvessel)thaninaconventionalcapillarytubeviscometer.(RedrawnfromLevyMN,ShareL.Circ Res. 1953;1:247.)9010 1 2 3 4 5 6 7 8 7030 50 Hematocrit ratio Relative viscosity Hind leg Capillary tube viscometer
Rheologic Properties of Blood
The viscosity of a given newtonian fluid at a specified temperature stays constant over a wide range of tube dimensions and flows. However, for a nonnewtonian fluid such as blood, viscosity may vary considerably as a function of tube dimensions and flows. Therefore, the term viscosity does not have a unique meaning for blood. The term apparent viscosity is frequently used for the derived value of blood viscosity obtained under the particular conditions of measurement.
Rheologically, blood is a suspension of formed elements, principally erythrocytes, in a relatively homogeneous liquid, the blood plasma. Because blood is a suspension, the apparent viscosity of blood varies as a function of the hematocrit (ratio of the volume of red blood cells to the volume of whole blood). The viscosity of plasma is 1.2 to 1.3 times that of water. The upper curve in
Fig. 17.7 shows that the apparent viscosity of blood with a normal hematocrit ratio of 45% is 2.4 times that of plasma.
In severe anemia, blood viscosity is low. As the hematocrit increases, the slope of the curve increases progressively; it is especially steep at the upper range of erythrocyte concentrations (see
Fig. 17.7
For any given hematocrit, the apparent viscosity of blood depends on the dimensions of the tube used in estimating the viscosity.
Fig. 17.8 demonstrates that the apparent viscosity of blood diminishes progressively as tube diameter decreases to less than approximately 0.3 mm. The diameters of the blood vessels with the highest resistance, the arterioles, are considerably less than this critical value. This phenomenon
Fig. 17.7 also illustrates that the apparent viscosity of blood, when measured in living tissues, is considerably less than the apparent viscosity of the same blood measured in a conventional capillary tube viscometer.